This invention relates to acrylic organic coating monomers curable with ultraviolet light to polymerize into relatively hard and tough organic coatings.
Prior art coatings of this general type utilize as crosslinking agents acrylic or other unsaturated monomers, together with separate photoinitiators, for producing free radicals after ultraviolet radiation. Such free radicals produce the necessary polymerization and crosslinking. Among the known acrylic type monomers used in the art are the following: Tetrahydrofurfuryl acrylate; Ethylene glycol phthalate acrylate; Dimethylaminoethyl acrylate; Ethylhexyl acrylate; Hydroxyethyl acrylate; N-vinyl-pyrrolidone; Hydroxpropyl acrylate; and some well known photoinitiators include the following: Benzoin alkyl ethers; 4,4'-bis(diethylamino)benzophenone; Acetone and other ketones; Benzophenone; and Thioxanthenone. Some well known crosslinking agents, which may be resinous or non-resinous, include the following: Polyesters of maleic anhydride; Acrylic terminated polyurethanes; Trimethylolpropane triacrylate; Pentaerythritol triacrylate; Hexanediol diacrylate; and Neopentyl glycol dimethacrylate. Since these crosslinking acrylic materials may be too viscous for convenient application, monomers are added as reactive diluents to lower the viscosity of the coating formulation.
In the prior art known to us the two necessary steps of photoinitiation and crosslinking are accomplished by at least two separate compounds of the types described above. The present invention seeks to provide both the photoinitiator and the crosslinking agent in one molecule. Formulations using this invention are believed to be simpler than prior art formulations. Another advantage of the new formulation is a faster cure rate.
As a possible explanation for the faster cure rate we consider the fate of a photoinitiator molecule after it has been activated by ultraviolet light, formed one or more free radicals, and has initiated polymerization. When the photoinitiator is a separate compound, as in the known prior art, the photoinitiator becomes an inactive tail on the polymerizing chain. The tail end of the chain containing the photoinitiator residue dangles freely without contributing to the crosslinking and therefore without contributing to the strength or coherence of the network. However with our invention, the photoinitiator (or fragment) at the chain end still contains at least one acrylic moiety which can continue the polymerization or crosslinking reaction and therefore can develop a strong coherent polymer network sooner.
The invention concerns a method of forming a polymeric coating on a substrate by irradiating a monomer coating material with ultraviolet light. The monomer coating may include as the sole reactive crosslinking agent an acrylic benzophenonetetracarboxylate having the following general formula: ##STR1## In the above general formula R.sub.1 are acrylic residues from alcohols such as 2-hydroxyethyl acrylate, and R.sub.2 are acrylic residues from epoxies, such as 2,3-epoxypropyl acrylate.
The above compound, on exposure to ultraviolet light, produces free radicals, which then initiate polymerization in surface coatings and inks.
The photoinitiator-crosslinking molecule of our invention (generalized structure shown above) is prepared in two steps. Starting with 3,3',4,4'-Benzophenonetetracarboxylic Acid Dianhydride (BTDA), we first alcoholize one mole of BTDA with two moles of unsaturated alcohol(s). The result is an intermediate di-ester/di-acid, shown below. ##STR2##
Second, we react the two remaining acid groups on the intermediate compound with two moles of epoxy(s), to form .beta.-hydroxyester linkages. The result is the previously described tetra-ester of BTA (acrylic benzophenonetetracarboxylate).
The manufacturing conditions for these two steps must also inhibit the free-radical reaction of the unsaturated compounds using appropriate inhibitors. The acid-epoxy reaction may be catalyzed for faster rates. Progress of the reactions may be conveniently followed by determining its acid number; dioxane is one useful solvent for this purpose.
The reaction between BTDA and the alcohol(s) preferably should be completed before adding any epoxy. The reaction between BTDA and alcohols occurs slowly at room temperatures and should be accelerated by higher temperatures and by suitable well-known catalysts. Generally, temperatures above 40.degree. C. are required to dissolve the components and to complete the alcoholisis reaction within a few hours.
Between 0% and 80% by weight of BTDA of an inert polar solvent (such as N-methyl-2-pyrrolidone, dimethylacetamide, dimethylformamide, and the like) is helpful in dissolving the components and does not interfere with subsequent reactions or with curing by ultraviolet light. Usually the solvent need not be removed from the final product.
Since anhydrides are well known as curing agents for epoxies, the epoxy compound need not necessarily be added after the alcohol has completely reacted with BTDA: the epoxy may also be added before or simultaneously with the alcohol, but then one must provide for the side-reaction between the alcohol and the epoxy.
The acid-epoxy reaction occurs slowly at room temperature but may be accelerated by higher temperatures or by suitable well-known catalysts such as tertiary amines, acids, bases, etc. Tertiary amines seem especially useful because they (1) selectively catalyze the reaction of the epoxide with acids in preference to alcohols and (2) because they may be co-catalysts with benzophenone-type photoinitiators.
The choice and concentration of inhibitor is not generally critical. Hydroquinone and p-methoxyphenol (also called hydroquinone methyl ether) are useful inhibitors at concentrations between 0.01% and 0.7% by weight of acrylic monomers. Their effectiveness is increased by oxygen or air. Air should preferably be pre-dried to prevent hydrolysis of the anhydride by water vapor.